Polyethylene terephthalate has been used for various products such as fibers and films, due to its excellent heat resistance, chemical resistance, electric characteristics, etc. However, in the case of using it for injection-molded plastic products, the molding work involves many limitations due to its peculiar crystallization behavior. Thus, it has been found to be seriously restricted in molding applications in spite of its excellent characteristics, because polyethylene terephthalate is an essentially crystalline polymer with a slow crystallizing rate.